In printer systems wherein an intensity-modulated beam of radiation (e.g., a laser beam) repeatedly scans a moving recording element to record image information, pixel-by-pixel, the velocity at which the recording element moves must be extremely uniform to produce high quality images. This also applies where the image information is being applied to a moving recording element line-by-line, such as with a linear array of light emitting diodes. If the velocity at which the recording element moves varies while the rate at which the image information is conveyed remains the same, there will be either a crowding together or spreading apart of lines of image information. This artifact, known as banding, causes a degradation in image quality.
In commonly assigned U.S. application Ser. No. 07/688,004, filed on Apr. 19, 1991 in the name of Kevin M. Johnson, entitled POSITION CONTROL APPARATUS FOR TRANSFER DRUM IN ELECTROSTATOGRAPHIC PRINTER/COPIER, there is disclosed an electrophotographic apparatus in which a scanning laser beam imagewise discharges an electrostatic charge on the surface of a photoconductive drum, leaving an electrostatic latent image. The electrostatic latent image is developed with colored toner particles from one of several development stations to create a transferable color toner image on the outer surface of the photoconductive drum. The toner image is transferred to a receiver sheet at a nip formed between the photoconductive drum and a transfer drum. The transfer drum is internally heated and its outer surface is urged into contact with the photoconductive drum surface at a relatively high force (e.g., 300-500 pounds). When the leading edge of the receiver sheet enters the nip, it suddenly retards the rotation of the photoconductive drum, causing a sudden increase in the torque (force) required to rotate the drum at constant speed. Similarly, when the trailing edge of the receiver sheet leaves the nip, the drum torque is suddenly decreased, causing a temporary increase in drum speed. If the laser is recording image information at the time of these "torque spikes", variations in line spacing will occur, causing the above-described banding artifact to appear. Of course, this artifact also appears as a result of any variation in velocity (sometimes known as "flutter") of the recording element.
The above-mentioned velocity fluctuations have a particularly adverse impact on the quality of prints produced by multicolor printer systems. In the Johnson apparatus, several color separation toner images are superimposed on each other on the receiver sheet to form a multicolor image. If the lines of image information are not uniformly spaced apart for each color separation image, color misregistration will result in the composite image.
Heretofore, various approaches have been proposed and used for uniformly driving the recording element in a printing system. They include direct drive systems in which, e.g., a motor shaft is directly coupled to the axle of a photoconductive drum, and indirect drive systems in which various means are provided for effecting a reduction in speed between the motor shaft and the driven element. Such speed-reducing means include compliant belts, chains and various types of gear trains. A speed reduction between the motor shaft and the recording element is employed to: (1) reduce the torque required of the motor; (2) reduce the current required; (3) run the motor in a more efficient state; and (4) allow for more uniform control of the motor, as motor speed uniformity is easier to obtain when the motor is running faster.
One approach to minimizing the effect of motor speed variations is to attach a large flywheel to the drive shaft of the motor used to advance the recording element or to the recording element itself. This has the effect of increasing the kinetic energy stored in the system when in operation. When the system experiences negative torque, the flywheel gives up some of its kinetic energy to assist in maintaining a constant motor speed. When a positive torque is induced on the motor, the large moment of inertia of the flywheel retards an increase in velocity of the motor. Such a system is shown in U.S. Pat. No. 4,935,778 to Mochida, issued on Jun. 19, 1990. This method works well for systems where disturbances, such as torque spikes, are minimal, but would do little to eliminate the flutter induced by large torque spikes. The increased inertia serves to lower the natural frequency of the system, which is undesirable because it makes servo-control of the driven element difficult.
A problem with traditional methods of speed reduction is that they introduce velocity errors into the system due to geometric inaccuracies, such as tooth-to-tooth errors in a gear-drive system and run-out of rotating elements in a compliant belt system. Further problems with some of these approaches are that they introduce compliance into the system, which is undesirable because: (1) a driven element will change velocity under the influence of a disturbance, such as a torque spike imposed upon a photoconductor drum; (2) after the disturbance is removed, the velocity will oscillate, with the magnitude of the oscillation being directly proportional to the compliance; and (3) the resonant frequency of the system is reduced when compliance is introduced, which makes servo-control of the driven element difficult. One other problem encountered in some reduction schemes is that backlash is introduced into the system, which makes servo-control of the driven element difficult or impossible.